Same-Origin Policy

Outcome: Design or review Same-Origin Policy for this system while keeping data correct, secure, and easy to operate. User context: A script on `reports.example` cannot read a user's inbox at `mail.example`. The browser can still send cookies there. A planned cross-origin Analytics API returns one exact `Access-Control-Allow-Origin` and no credentials. Window messages are accepted only after checking `event.origin` and the message shape. Technical concept or control: Same-Origin Policy. Why it fits: Treat each origin as a basic browser safety boundary. When sharing is needed, create clear CORS or `postMessage` access with the least permission. Do not use it when: SOP does not stop all cross-site requests, navigation, embedding, CSRF, or side channels. Its main job is to limit reading and interaction. Engineering requirements: Define inputs, outputs, contracts, and rules that must always hold. Set authorization and input checks. Define failures, timeouts, retries, one-time effects, data moves, and compatibility limits. Operations: Add useful signals and logs that hide sensitive data. Set capacity limits and write an operator guide. Test recovery and rollback. Acceptance criteria: Contract and failure-path tests pass. Data and authorization rules still hold during retries and partial failures. Check performance limits, monitoring, recovery, and rollback. Evidence and limits (evidence boundary): The standard covers only the listed scope. It does not choose the visual style, product decision, or Vibulary example. Unknowns to confirm: Where the code runs, data stores, traffic shape, consistency needs, and the security boundary. Also confirm the compatibility period, service goals, and rollback limits.